Hydraulic testing means



1967 P. B. HINDE 3,333,417

HYDRAULI C TEST ING MEANS Filed Oct. 1, 1964 PHIL) P BENJA HN HINDEUnited States Patent 39,518/ 63 4 Claims. (Cl. 60-545) This inventionrelates to hydraulically powered physical testing apparatus and moreespecially, but not exclusively, to machines for use in thedetermination of load/deflection or stress/strain characteristics ofmaterials.

Certain hydraulic testing machines have been proposed which operate bydelivering hydraulic liquid to a ram at a constant volumetric rateindependent of the pressure produced in said liquid. In consequence aspecimen being acted upon by the ram is continually and increasinglystrained and measurements can be intermittently or continually taken ofthe load produced on the specimen and of the resulting deflection fordetermination of the characteristics of the specimen and its material.

It has been found that in general, although the hydraulic liquid may bedelivered to the ram at a constant volumetric rate, the specimen is notstrained at a corresponding rate because the elasticity in the ram andalso in the liquid supply system to the ram comprising for exampledelivery pipes, and devices for controlling and measuring the pressurein the liquid, provides a varying capacity for the liquid. In additionthe elasticity of the liquid itself also results in a varying liquidvolumes. For this reason, the testing machine has in practice a finitestiffness.

Further, the deformed elastic parts and the liquid store potentialenergy which is released when the pressure of the liquid falls and thusmaintains said pressure at a higher value than would exist if the supplysystem and liquid were completely non-elastic, i.e., if the testingmachine were rigid or infinitely stiff. Thus, if during a test aspecimen reaches its yield point and commences to deflect at anincreasing rate so that the pressure in the liquid falls, themaintenance of pressure by the released potential energy undesirablycontinues the increas ing deflection without control, and a constantdeflection rate is lost.

It is an object of the invention to provide a stiffness adjuster for usein a testing machine whereby the natural stiifnas of the machinedetermined by its construction and the hydraulic liquid used can beadjusted to a higher value.

The invention consists in a stiffness adjuster for use in hydraulicallypowered testing apparatus comprising a ram and a supply system to supplyhydraulic liquid to said ram, said stiffness adjuster comprising meansresponsive to increasing liquid pressure in said system and ram fordecreasing the hydraulic liquid capacity of the supply system and ram.

A typical example of the practical relationship of the invention willnow be described with reference to the accompanying drawings in which:

FIGURE 1 is a diagrammatic sectional view of a stiffness adjuster inaccordance with the invention; and

FIGURE 2 is a diagrammatic sectional view of a conventional hydraulictesting machine.

FIGURE 1 shows a stiffness adjuster 5 which comprises two cylinders 6, 7secured together in co-axial alignment by tie rods 8 extending betweenflanges 9 and 10 one on each of the cylinders 6, 7. The cylinders 6, 7contain co-axial pistons 11 and 12 respectively of differentcrosssectional areas joined together so that as one piston 11 or 12sinks more deeply into its cylinder so the other piston 12 or 11partially emerges from its 3,333,417 Patented Aug. 1, 1967 cylinder. Acompressed helical spring 13 bears against the end 14 of the largerpiston 11 between the cylinders 6, 7, encircles the smaller piston 12,and at its other end abuts the inner end of cylinder 7. Each cylinder 6or 7 is provided with an inlet/outlet passage 15, 16 respectively forconnection into the hydraulic circuit of a testing machine. The inlet/outlet passage 15 leading from the cylinder 6 includes a stop valve 17.

In use the stiffness adjuster 5 is connected into the hydraulic testingmachine as shown in FIGURE 2. The testing machine comprises a pump 18for supplying hy draulic liquid from a reservoir 20 to the main chamber19 of a control device. In said device an adjustable needle valve 21controls communication between chamber 19 and the supply system 22. Thesupply system 22 includes a delivery pipe 23 leading to a hydraulic ram24 for straining a specimen (not shown), and a branch pipe 25 leadingfrom the delivery pipe 23 to a secondary chamber 26 housing acompression spring 27 bearing against one end 28 of a control piston 29.The other end 30 of the piston 29 is for closing a leak passage 31between the main chamber 19 and the reservoir 20.

When the testing machine is straining the specimen in a test, thepressure of the liquid in the secondary chamber 26, which is equal tothe pressure in the delivery pipe 23, plus the pressure of the spring27, acts on the end 28 of the control piston 29, whereas the pressure ofthe liquid in the main chamber 19 acts upon the end 30 of the controlpiston 29. Consequently when the forces on the .two ends of the controlpiston 29 equalise the control piston 29 allows surplus liquid outputfor the pump to leak back to the reservoir 20 through the passage 31,and a constant pressure difference corresponding to the forces of thecompression spring 27 exists between the liquid parts acting on thecontrol piston 29, and so will also exist across the needle valve 21 sothat liquid is delivered to the supply system 22 through the valve 21 ata constant, rate for any given setting of the needle valve 21. If thepressure in the supply system 22 should suddenly drop (more especiallydue to a sudden yielding in the specimen), the leak passage 31 will befurther opened until the original pressure difference is restored.

The inlet/outlet passage 15 of the cylinder 6 is connected to the mainchamber 19, and the inlet/outlet passage 16 is connected to the deliverypipe 23 so that the constant pressure difference also exists between theliquid parts within the cylinders 6, 7. However, said liquid parts areacting on pistons 11, 12 of different crosssectional areas so that, asthe pressures increase, the larger piston 11 will progressively move outof its cylinder 6 against the action of the spring 13, and the smallerpiston 12 will move into its cylinder 7 and thus displace some of theliquid from the latter into the supply system 22, thereby effectivelydecreasing the capacity of said system. As the pressures fall, thereverse changes take place and the capacity of the supply system 22 isincreased. Thus these changes in capacity of the supply system 22 arethe reverse of the capacity changes taking place through the deformationof elastic parts in the supply system 22 and the liquid under changingpressure. Consequently the stiffness of the machine is increased, andmay be increased to infinity if exact adjustment is arranged.

The amount of adjustment required of the stiffness adjuster 5 can beprovided by calculating the change of capacity needed to be produced bythe adjuster from zero to maximum pressure in the supply system 22,arranging for the smaller piston 12 to effect this change over its totaltravel, and choosing a spring 13 and diameter of the larger piston 11 toproduce this travel over this total pressure range. Thereafteradaptation of the adjuster can be made by substitution of a differentspring 13 to alter the amount of adjustment produced by the adjuster 5and hence produce a different stiffness.

It is found that, when the testing machine described is used completewith the stiffness adjuster, a specimen being tested is strained at arate corresponding to the rate of liquid delivered to the supply system22 to a greater extent than Without the adjuster 5, and the release ofpotential energy occurring during decreasing pressure tends to maintainthe pressure in the liquid to a lower extent than without the adjuster5.

To improve the response to the adjuster 5 it may include means forpromoting angular oscillation of or rotation of its pistons 11, 12 tominimise frictional effects.

Whereas the stiffness adjuster 5 has been described as applied to ahydraulic testing machine, said adjuster is also applicable to otherhydraulic testing apparatus such as hydraulic testing rigs or frames inwhich the testing means are not self-contained but depend structurallyupon a part or parts of buildings or the specimens themselves.

I claim:

1. In a hydraulic testing machine of the type comprising a hydraulicram, a hydraulic system having a chamber receiving said ram andconnected to receive hydraulic liquid from a source for displacing saidram against a load applied thereto, and means for maintaining asubstantially constant pressure differential between said source andsaid chamber for delivering said liquid to said chamber at a constantvolumetric rate independent of variations of liquid in said chamberresulting from the magnitude of the load applied to said ram, meansresponsive to the liquid pressures at said cource and said chamber forcompensating for the elasticity of the liquid in said system and for theelasticity of ram and the component parts defining said system byinversely varying the volumetric capacity in said system with respect tothe liquid pressure in said chamber.

2. The hydraulic testing machine defined in claim 1 wherein saidcompensating means comprises a compound piston having (a) a largediametered section received in a first cylinder section and connected byfirst passage means directly to said source and (b) a smaller diameteredsection received in a second cylinder section and connected by secondpassage means directly to said chamber, said compound piston being (a)displaceable by increase of liquid pressures in said chamber and at saidsource to displace liquid from said second cylinder section into saidsystem and (b) displaceable by decrease of liquid pressures in saidchamber and at said source to enable liquid in said system to flow intosaid second cylinder section.

3. The hydraulic testing machine defined in claim 2 1,467,522 9/1923Amsler -52 2,102,865 12/1937 Vickers 6052 MARTIN P. SCHWADRON, PrimaryExaminer.

ROBERT R. BUNEVICH, Assistant Examiner.

1. IN A HYDRAULIC TESTING MACHINE OF THE TYPE COMPRISING A HYDRAULICRAM, A HYDRAULIC SYSTEM HAVING A CHAMBER RECEIVING SAID RAM ANDCONNECTED TO RECEIVE HYDRAULIC LIQUID FROM A SOURCE FOR DISPLACING SAIDRAM AGAINST A LOAD APPLIED THERETO, AND MEANS FOR MAINTAINING ASUBSTANTIALLY CONSTANT PRESSURE DIFFERENTIAL BETWEEN SAID SOURCE ANDSAID CHAMBER FOR DELIVERING SAID LIQUID TO SAID CHAMBER AT A CONSTANTVOLUMETRIC RATE INDEPENDENT OF VARIATIONS OF LIQUID IN SAID CHAMBERRESULTING FROM